In recent years, high rotational accuracy is being required to a rotation driving part, such as an optical deflection scanner, for example, of a magnetic recording apparatus or a laser beam printer along with a high rotational speed. In order to rotate a precision motor to which such high rotational speed and high rotational accuracy are required at a higher speed, employment of a gas bearing (hydrodynamic gas bearing) for the rotation driving part is proposed. In the rotation driving part employing this gas bearing, air is forcibly introduced into at least a clearance between a radial gas bearing body and a rotator when the rotator rotates. Thus, the air pressure in the clearance is increased, and the rotator rotates at a high speed through the gas bearing. Thus, maintenance of the rotational accuracy is expected also during high-speed rotation, by employing the gas bearing.
In the aforementioned radial gas bearing, a wedge clearance is formed by eccentricity of a shaft body in the bearing body, as shown in "Gas Bearing" by Shinichi Tohgo, Kyoritsu Shuppan (1984), for example. Pressure is generated when air passes through this wedge clearance since the air is compressed. Thus, it becomes possible to support the shaft body and the bearing body in a non-contact manner.
According to "About Whirl of Gas Bearing" by Atsunobu Mori, pp. 481 to 488, "Lubrication" Vol. 20, No. 7 (1975), however, an unstable phenomenon called "whirl" (H/W) is observed in a cylindrical journal bearing when set in an unloaded state such as the case of supporting a vertical shaft or the like. This phenomenon is such that the shaft is pressed against the bearing surface by centrifugal force to whirl in the interior of the bearing. In the cylindrical journal bearing, the bearing center and the rotation center deviate from each other by a static load to generate pressure in one portion and bring stable rotation. In case of employing the cylindrical journal bearing for a vertical structure, i.e., a structure supporting a vertical shaft or the like, however, the bearing is set in an unloaded state and hence a pressure-producing point changes by disturbance and rotation becomes unstable.
In case of applying the aforementioned hydrodynamic gas bearing to a rotation driving part of a magnetic recording apparatus such as a hard disk driver or a laser printer, the aforementioned unstable factor must be eliminated since the positional accuracy of the rotator is regarded as important.
Accordingly, there is proposed in Japanese Patent Publication No. 4-21844 (corresponds to Japanese Patent Laying-Open No. 58-224324) that generated pressure increases by forming shallow grooves mainly on a side of a shaft body, serving as a rotator, into which gas flows by rotation circumferentially in equal distribution, to improve whirling stability in high-speed rotation, i.e., to prevent a whirl phenomenon.
Further, there is proposed in Japanese Patent Laying-Open No. 8-312639 means of forming at least three grooves extending in the axial direction on a shaft body circumferentially in equal distribution and controlling the groove shape, thereby improving whirl stability in high-speed rotation and preventing a whirl phenomenon.
According to experiments by the inventors, however, it has been proved that there are the following problems in case of forming grooves on a shaft body in accordance with the aforementioned proposals, although a whirl phenomenon in high-speed rotation can be suppressed:
FIG. 11 is a cross-sectional view of a shaft body. As shown in FIG. 11, three portions of grooves 13 are formed on the outer peripheral surface of the shaft body 1. In this case, the shape of the grooves 13 has a laterally symmetrical shape in the circumferential direction of the shaft body 1. The outer peripheral surface of the shaft body 1 has portions which are circumscribed with a circle having a diameter Dout and inscribed with a circle having a diameter Din. The average diameter of the shaft body 1 is shown by Dm.
When forming a hydrodynamic gas bearing structure with the shaft body 1 having such a cross section, it is possible to suppress a whirl phenomenon in high-speed rotation. However, dispersion results in the rotational frequency at the time of rotating/starting the shaft body 1 to shift from such a state that the shaft body and a bearing body are in contact with each other to a non-contact state, or the rotational frequency when the shaft body and the bearing body shift from a non-contact state to a contact state when reducing the rotational speed from a state of stationary rotation of a high speed to stop the rotation, i.e., "floating rotational number". In particular, there has been such a problem that this floating rotational frequency may extremely increase. Thus, there has been such a problem that it is impossible to shift the shaft body and the bearing body from the contact state to the non-contact state at a low rotational frequency but the shaft body and the bearing body are continuously in contact with each other at a relatively high rotational frequency in starting or stoppage of rotation thereby causing abrasion powder. In addition, there has also been such a problem that galling is caused between the shaft body and the bearing body by the abrasion powder.
It has been proved that the aforementioned dispersion of the floating rotational frequency is correlated with the fact that the outer peripheral shape of the shaft body deviates from a complete round as shown in FIG. 11. Namely, it has been proved that the aforementioned dispersion of the floating rotational frequency is large as the out-of-roundness (=(radius of circumscribed circle: Dout/2)-(inscribed circle radius: Din/2)) of the outer peripheral portion of the shaft body 1 excluding the grooves is large. In manufacturing of the shaft body, there has been such a problem that a probability in which a shaft body whose floating rotational frequency is high is manufactured increases, and the manufacturing yield lowers as a result.
An object of the present invention is to provide a hydrodynamic gas bearing structure which is capable of reducing such frequency that a floating rotational frequency increases in starting or stoppage of rotation.
Another object of the present invention is to provide a hydrodynamic gas bearing structure which is capable of shifting a floating rotational frequency in starting or stoppage of rotation to a low rotational frequency side.
Further, still another object of the present invention is to provide a hydrodynamic gas bearing structure which is capable of further effectively preventing an wear phenomenon in starting or stoppage of rotation by reducing such frequency that the floating rotational frequency in starting or stoppage of rotation increases and shifting the floating rotational frequency to a low rotational frequency side.
A further object of the present invention is capable of effectively preventing an wear phenomenon in starting or stoppage of rotation even if the out-of-roundness of a shaft body is large (bad) and to further improve the manufacturing yield of the shaft body in a hydrodynamic gas bearing structure.